Pressure-equalizing steam control valves for steam turbines, which are designed as single-seat or double-seat valves and are actuated by means of oil hydraulic drives, have been known.
The double-seat valve is pressure-equalized as a consequence of the back-to-back arrangement of two valve disks or cones, so that no appreciable opening forces are generated from the steam pressure in the closed state of the valve as a consequence of the minimum differential pressure acting on the valve.
Since the hydraulic drives may consequently be smaller, the force for displacing the oil during the closing process is weaker as well, and since the cross section of the valve spindle may also be smaller because of the weaker adjusting forces, the spindle lift is also smaller in the opened state of the valve, and the pretension of the closing spring may be weaker. This means that the forces necessary for actuating a double seat valve are relatively weak.
However, the double seat valve is almost never sealed for technical reasons. This is due, on the one hand, to the redundancy in the dimensioning of the distance between the two valve seats, and, on the other hand, to the thermal expansions of the valve housing and the valve spindle with the two integral valve disks not being exactly equal.
In the prior-art single-seat valve of piston design, the same pressure that prevails behind the valve when viewed in the direction of flow is admitted to the rear side of the valve, as in the present invention.
However, the cross-sectional area of the valve spindle is eliminated for a complete pressure equalization due to the necessity to connect the valve spindle on the rear side of the valve cone. As in the double seat valve, the cross section of the valve spindle may be smaller than in the case of a valve without pressure equalization. The closing spring may also be installed with a weaker pretension because of the resulting reduction in the spindle lift, i.e., the forces necessary for actuating the valve are weak, as in the double-seat valve.
The valve cone designed as a piston is necessarily sealed against the valve core designed as a bushing. However, the steam leaking via this sealing ring (sealing rings) flows to the blading of the turbine via the pressure-equalizing holes in the valve cone. This may lead to an unintended startup of the turbine at high steam pressures.
The oil-hydraulic drives of the prior art steam control valves are now always arranged outside the area of hot turbine parts for reasons of fire safety. This is to avoid the risk of a oil fire (hydraulic fluid fire). This means that valves and drives must be arranged under the turbine, or that if the valves are arranged above the turbine, the hydraulic drives must be at a remote location and provide their function via levers and deflections.